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Alpha fetoprotein is a type of protein that is produced in large amounts by fetal cells. By the time a child is 1 year old, the levels of alpha fetoprotein have dropped considerably. The levels of this protein are abnormally high in certain types of abnormal pregnancies, so a test to determine protein levels is a useful screening or diagnostic tool, particularly when combined with other diagnostic blood tests.
The simplest alpha fetoprotein test is a blood test to detect the levels of the protein in a pregnant woman’s blood. This blood test is carried out, along with tests for certain hormones, between the 15th and 20th weeks of pregnancy. The results of the blood tests, along with details such as the age of the pregnant woman and the presence or absence of pregnancy risk factors, provide information that is used to determine how likely it is that the fetus might be born with one or more birth defects.
This test can be used to estimate the risk of several conditions and birth defects. For example, in combination with blood tests for the hormones estrogen, human chorionic gonadotropin and inhibin A, the fetoprotein blood test can detect Down syndrome approximately 80 percent of the time. This test also can be used to detect neural tube defects, a type of abnormal development of the brain or spine.
The levels of alpha fetoprotein begin to rise in approximately the 14th week of pregnancy and continue to rise until the third trimester. One to two months before birth, the protein levels taper off and then begin to decrease. In a pregnant woman, normal protein levels can be as much as 2.5 times what is considered normal for a woman who is not pregnant.
Abnormally high or low protein levels have several implications. One is that the estimated gestational age of the baby has been miscalculated, perhaps meaning the doctor must revise his or her estimated delivery date. High protein levels might also mean that the pregnant woman is carrying twins or triplets. An abnormally high level of protein can indicate that the fetus has a neural tube defect or an abdominal wall defect called an omphalocele, in which some of the abdominal organs are located outside the body. It might also mean that the fetus has died. An abnormally low level of protein can indicate that the fetus has Down syndrome.
The measurement of alpha fetoprotein levels is not a test that is accurate all of the time. It is possible for protein levels to be unusually high or low during a normal pregnancy. For this reason, the test is considered a starting point for diagnosis rather than an end point.